Investigation of the surface integrity and fatigue strength of Inconel718 after wire EDM and machine hammer peening

Highly stressed turbine blades made of Inconel718 (DIN NiCr19NbMo (2.4668); ASTM B637; UNS N07718) have to satisfy stringent requirements regarding durability and reliability. The impact of surface integrity on the fatigue strength of the workpiece material has proven to be of significant importance. An optimization of the surface integrity after wire EDM of Inconel718 can be achieved using the surface finishing process machine hammer peening. Machine hammer peening is an incremental forming process which has the potential to induce residual compressive stresses and strain hardening while smoothing surface asperities. As the effect of machine hammer peening on surface integrity has not yet been completely investigated, this paper deals with its description and modeling of the unknown impact of machine hammer peening on the fatigue strength of Inconel718. Wire EDM and machine hammer peening parameters maximizing the fatigue strength are presented. Experimental measurements are performed to quantify the surface integrity, especially surface roughness, compressive residual stresses and strain hardening, after wire EDM and subsequent machine hammer peening. The evaluation of the effect of surface integrity on the fatigue strength is carried out using a bending fatigue strength testing machine. Additionally, a finite element model is developed to analyze the surface integrity under load and to reveal the effects of difficult-to-measure properties on the fatigue strength. Based on the new knowledge gained during this work, a model to predict the fatigue strength using similitude theory is presented. Along with the previously analyzed correlations between surface integrity and fatigue strength, this allows a constructive setting of machine hammer peening parameters. Thereby, a significant new contribution to knowledge about the influence of machine hammer peening on surface integrity and fatigue strength of Inconel718 is made. The presented approach can be easily transferred to other materials and workpiece geometries.